Gemcitabine is a new anti-metabolite that has a unique mechanism of action; it is not cycle-specific or phase-specific. Therefore, we hypothesize that it may be effective in solid tumors that show resistance to other classes of chemotherapeutic agents. In studies of adults, gender differences in drug metabolism have been shown. We hypothesize that there will be similar gender differences in the pharmacokinetics of gemcitabine metabolism. Age-specific labeling is needed for use of this potentially active agent in pediatric patients. The specific aims of this study are to: 1) Determine the maximum tolerable dose of gemcitabine when given (1) as a 30 minute intravenous infusion weekly for 3 weeks, followed by one week during which no gemcitabine is administered, and (2) as a 30 minute intravenous infusion weekly for 2 weeks, followed by one week during which no gemcitabine is administered 2) Determine the toxicities of gemcitabine when given on these schedules. Determine the pharmacokinetic behavior of gemcitabine in male and female pediatric patients. 3) Determine the anti-tumor activity of gemcitabine within the confines of a Phase I study.